A. E. W. Mason 
They Wouldn’t Be Chessmen [EPUB ebook] 

Nahendra Nao, heir to the Maharajah of Chitipur, unwisely lets Elsie Marsh of the Casino de Paris wear his ancestral pearls, which react badly to her skin. In order to restore their lustre his secretary, Major Scott Carruthers, hires a beautiful, down-on-her-luck opera singer, Lydia Flight, to wear them while they heal. They take a houseboat on the Seine near Caudebec-en-Caux, and while there make the acquaintance of Julius Ricardo. When the pearls are stolen Ricardo teams up with his old friend Inspector Gabriel Hanaud to solve the mystery.